I prefer to use a thinner thread for piecing, such as Superior's Masterpiece (50 wt). For quilting I may go to a little heavier thread so the quilting shows (except when doing SID or tiny backfills, then I will stay with the thinner thread). Using a thinner piecing thread takes up less room in the seam allowance so helps in achieving accurate 1/4" seam allowances.
Andi